Heat one tablespoon of the oil in a large saucepan over medium heat. Add the mushrooms and garlic, and cook them for about 5 minutes.

Transfer the mushrooms to a large bowl, and add all remaining ingredients. Knead the mixture together with the hands.

Divide the black bean mixture into 6 patties.

Heat remaining two tablespoons of the oil in a large skillet over medium heat. Add the burgers, and cook them for about 3 to 4 minutes per side, or until the burgers turn golden brown on both sides.

Serve the burgers on the slices of whole grain bread, and top them with the mustard, sliced vegetables and seeds.

Note: The buckwheat flour could be replaced by the almond flour (finely ground almonds) and the sweet potato could substitute for the carrot; just bake, peel and cut the potato into cubes, before adding it to the black bean mixture.